What is n+, p+, p doping?

I'm reading an article that has n+, p+, and p doped silicon wafers. http://iopscience.iop.org/0953-8984/10/44/001"
I hadn't heard of n+ or p+ before, just n and p. I noticed in the article that n+ and p+ had really low resistivites (10^-2 Ohm-cm) and p had a resistivity of about 10 Ohm-cm.

So does the plus sign just mean it is heavily doped or is there a different meaning to the + sign?

Yes, the plus sign refers to "extrinsic" doping such that the material is highly conductive. This is used typically in the outer layers of p-i-n diodes. By contrast, the i layer is "intrinsic," or undoped and highly resistive.

I usually use n+ and p+ for Si that is doped heavily enough that the resistivity is down in the range of milliOhm-cm. When it's greater than an Ohm-cm, I call it n or p type. I think that's not completely inconsistent with the literature.